A conventional seat belt apparatus is known which is configured to have a belt retractor including an electric motor to make a spool be driven and rotated so that a seat belt is retracted and drawn out. When a seat belt is retracted and drawn out by the retractor having an electric motor, a failure in the electric motor makes it impossible for the seat belt apparatus to deliver its intended function. Technology for diagnosing such a failure is disclosed in Japanese Patent application Laid-open No. Hei 11-170966.
According to this disclosed conventional technology, a voltage having specified waveforms is applied to an electric motor and waveforms of the current through the motor are used for diagnosing.
In the technology disclosed in the patent reference, the seat belt apparatus is so configured that a seat belt does not cause a feeling of abnormality or discomfort to a vehicle occupant by lowering the voltage to be applied to the electric motor as much as possible or by shortening the time during which the voltage is applied. However, when the above failure diagnosing operation is actually performed while the vehicle occupant is fastening the seat belt, there is a possibility that the vehicle occupant will have a feeling of abnormality or discomfort. Furthermore, if this failure diagnosis is carried out at bad timing despite actual necessity of driving of the electric motor (that is, when an retracting operation or drawing operation is required), there is a possibility that the desired operation of the motor are adversely affected. In other words, there is a possibility that not only a vehicle occupant fastening a seat belt has a feeling of abnormality or discomfort but also an operation of the seat belt apparatus itself is adversely affected.
Thus, in view of the above, it is an object of the present invention to provide a failure diagnosing method for a vehicle seat belt apparatus which is capable of carrying out a failure diagnosis without causing a feeling of abnormality or discomfort to a vehicle occupant or adversely affecting an original operation of a seat belt and a vehicle seat belt apparatus having a failure diagnosing function to carry out the diagnosis.